The invention is based on a priority application EP 03 291 119.0 which is hereby incorporated by reference.
The present invention relates to an optical amplifier and an optical communication system which incorporate the method and device to adjust gain tilt of the optical amplifier.
Optical communication systems typically use wavelength-division multiplexing to increase transmission capacity. More specifically, a plurality of signal lights each having a different wavelength are multiplexed together into a wavelength division multiplexed (WDM) signal light. The WDM signal light is transmitted over a transmission line, and then demultiplexed at the other end of the transmission line so that the individual signal lights can be individually received. The transmission line is usually a single optical fiber.
An optical amplifier is typically used in such an optical communication system to amplify the WDM signal light, since an optical amplifier has a relatively broad band.
More specifically, the broad band of the optical amplifier allows each of the individual signal lights in the WDM signal light to be amplified.
Generally, an optical amplifier includes an optical amplifying medium, such as an erbium-doped fiber (EDF). The WDM signal light travels through the optical amplifying medium. The optical amplifier also includes a light source, such as a laser diode, which provides pump light to the optical amplifying medium. The pump light causes the WDM signal light to be amplified as the light signal travels through the optical amplifying medium. Repeaters, each have an optical amplifier, are typically interposed into the transmission line to transmit a WDM signal light over a long distance.
Moreover, the gain of an optical amplifier is dependent on the wavelength of the amplified signal. This dependence is defined as the “gain tilt” of the optical amplifier. Therefore, when a WDM signal light is amplified by the optical amplifier, each of the individual signal lights multiplexed together may be amplified with a different gain. This gain tilt of the optical amplifier must be considered when using an optical amplifier to amplify a WDM signal light.
Therefore, the gain tilt of an optical amplifier should be monitored or controlled when using the optical amplifier in an optical communication system, which used wavelength division multiplexing.
Actually the tilt of an optical amplifier is controlled by adjusting a VOA (variable optical attenuator) in the interstage of the amplifier. As described also in FIG. 2. In a long haul system using more than 10 amplifiers, the tilt at the end of the system can be very important (more than 10 dB) and the system performance is degraded.
The feedback signal for a VASC (Variable Slope Compensator) in the interstage of the EDFA is still not yet clearly defined.
The spectrum flatness of the optical amplifier is adjusted by measuring the gain of the amplifier. If the amplifier works at his nominal gain, the gain is spectrally flat. Power measurements at the input, output and interstage of the amplifier allow to adjust the gain of the amplifier to his nominal gain by changing the attenuation of the VOA.
This technique allow to reduce the tilt to approximately 1.5 dB per amplifier (due to the uncertainty of the power measurement at the input/output/interstage of the amplifier). But in a very long haul system of approximately 1000 km (typically 10 to 15 amplifiers), the accumulated tilt can be as high as 20 dB. In the case of a standard EDFA, the amplifier gain is flat if the gain is set to is nominal value. The different tap couplers of the amplifier allow to measure the gain of the first and second stage, and the VOA is adjust to achieve the nominal value of the gain.
Different sources of unflatness can be noted:                Due to the inaccuracy of the photodiode, the gain is not exactly set to is nominal value.        Due to the non-uniform loss of the span between 1530 nm and 1562 nm, the spectrum at the input is not flat, so even if the gain of the amplifier is perfectly flat, a tilt is created. The difference of loss can be 1 dB between 1530 nm and 1560 nm for a long span of 100 km.        Due to the high power sent in the span, there is an energy transfer between the lower wavelength of the spectrum to the higher wavelength thanks to the Raman effect. If the input power is as high as 23 dB, the tilt can be higher than 2 dB for the C Band. (If the system uses C+L Band, the tilt can be higher than 3 dB).        
For this reason, some amplifiers are now designed to generate more gain at lower wavelength than at higher wavelength (in the case of the C-Band, the amplifier gain is higher at 1529 nm than at 1562 nm by 1 or 2 dB).
In order to avoid to sum the error of 1.5 dB for the tilt adjustment at each amplifier; the ASE noise is measured at the two extremities of the optical bandwidth of the amplifier. The VOA or the VASC (Variable Slope Compensator) at the interstage is adjusted in order to minimized the tilt on the ASE noise. The tilt on the ASE noise is linked with the tilt of the signal (because the NF of the EDFA is nearly flat on the bandwidth of the optical amplifier).
If the amplifier has been designed to generate more gain at lower wavelength than at the higher wavelength, the ASE noise measured at the lower wavelength has to be larger than the ASE noise at the higher wavelength.
One solution to adapt gain tilt over a WDM system is disclosed in U.S. Pat. No. 6,160,659. In detail an apparatus is disclosed which receives an input light having a spectrum, and determines a momental wavelength of the spectrum. The apparatus includes a decoupling unit, a weighting unit and a computation unit. The decoupling unit decouples a portion of the received input light, to provide a first signal representing the input light with the portion decoupled there from, and a second signal representing the decoupled portion. The weighting unit weights the second signal. The computation unit determines the momental wavelength from the power of the first signal and the power of the weighted second signal. An optical amplifier is also provided which determines the momental wavelength of an amplified light, and controls a gain tilt parameter of the optical amplifier in accordance with the determined momental wavelength, to reduce gain tilt. The momental wavelength is determined by measuring the ASE (Amplified Spontaneous Emission) of the fiber amplifier. The apparatus described in this prior art must work in a “full loaded” status to derive the parameter for momemtal wavelength. This means that all channels of a the wavelength multiplex must be active and loaded with signals. For a system without signals or with a limited use of several channels only the apparatus of U.S. Pat. No. 6,160,659 does not help.